1. Field of the Invention
The present invention relates to a wide area network (WAN), and more particularly, to such a network for the control of data processing devices that extend over a relatively large physical area using a modified carrier sense multiple access with collision detection (CSMA/CT) access method (or commonly referred to as Ethernet).
2. Description of the Related Art
Ethernet is the progeny of the standardized access methods used today, as more specifically documented in ANSI/IEEE Std. 802.3. However, due to the inherent propagation delays and characteristics of the collision detection process for independent broadcasting from each of the networked units, the application of these methods is limited to local area networks (LAN) and typically not beyond 100 feet using copper wire or typically 300 meters on fiber. Additionally, Ethernet is a peer to peer communication scheme whereby a device acquires communication privileges on a first come first serve basis. This scheme does not guarantee a device will be allowed to communicate in any given period of time.
A particularly useful application for a wide area network (WAN) is the connection of vehicular traffic control devices that are distributed over a relatively large geographical area, such as a city or a county, to a central location containing computer devices. Typically, traffic control systems communicate with the central location once every second, as a minimum. The present invention will permit the exchange of control signals to and from a polling or communications server to remote vehicular traffic control devices and in addition the more complex video signals will travel from the traffic control devices to the appropriate video signal receiving equipment. At least three thousand traffic devices can be controlled over a fiber optic network if the conventional Ethernet collision detection and random delay features are disabled until a client station is accessed. And this can be achieved at a considerable lower cost than using dedicated modems for electrical transmission over copper lines, or fiber optic communication lines.
Ethernet technology in its standard form is not acceptable as a communications scheme for traffic control systems for two main reasons. First, the peer to peer communication relationship among all of the field locations and the central location or host computer does not guarantee the exchange of data at the minimum rate expected (once per second).
Second, the maximum recommended distance for an Ethernet connection is 100 meters (328 feet) over copper lines and 2 kilometers (1.25 miles) over a fiber optic network. The present invention will permit the exchange of control signals and various other data signals from the host computer at a central location to each and every vehicular traffic control device each second regardless of the distance between the host and the vehicular traffic control device (or intersection).
A typical LAN Ethernet installation for more than one site, as shown in FIG. 1, includes a computer that acts as a server device, a hub, at least one computer workstation (two are shown in FIG. 1), communication servers (three are shown in FIG. 1) and possibly other devices (not shown). In the leftmost WAN network, two Asynchronous Transfer Mode (ATM) communication devices are connected via fiber optic cable. The ATM communication device is a conduit for multiple data paths over two fiber optic conduits, or other high speed media, among locations that are far apart. The ATM device is used to connect a remote LAN location to a central location or host computer. In this configuration the communication server sends a broadcast command to establish contact with each of the components connected to it. The communication servers include a device called an Ethernet NIC (network interface card) that provides the necessary hardware components needed to implement the Ethernet interface. In the leftmost connection shown in FIG. 1, a communication server""s NIC is connected to a similar NIC housed in the ATM device. The other communication server""s NIC is connected to compatible hardware in the stackable hub at the remote location. The hub sends the Ethernet frame to all workstations connected to it simultaneously. The simultaneous transmission is accomplished via the star network configuration. All of the workstations, after receiving a command will attempt transmission to the central servers, respectively. The station that is the fastest will be more likely to succeed in its transmission because it will not sense a carrier from any other device. The other workstations, when they sense a carrier of the responding workstation, start a random timer used to compute the delay for their respective next attempts to transmit. The workstation with the shortest random time delay will be the next one to transmit. This sequence will continue until all of the workstations have reported. This type of operation is referred to as peer to peer and non-deterministic communication. From time to time each server sends a broadcast command to see if any new locations have been added to the network. In addition to the broadcast situation from the server, each workstation can send data to the server or to another workstation whenever the line is not busy.
To use Ethernet technology for communications in a traffic system, the operation needs to be modified. The peer to peer relationship (non-deterministic) is made to be master-slave and deterministic. To do this the conventional Ethernet collision detection and random delay features are disabled. Through the use of special software drivers, the host computer becomes the only source of authorization for transmission from anywhere in the entire network. Only after a request for data is received does a location respond with an Ethernet frame transmission. Unlike the situation mentioned above whereby the server sends a broadcast command to learn what is connected to it, the servers used for a traffic control application will use a preprogrammed table containing the IP address of every device on the network. Whenever a new device is attached to the system, its address will be added to the table before it is connected to the network thus eliminating the need for broadcasting.
The present invention does not require the use of stackable hubs nor are the devices connected in a star configuration. A star configuration would require an enormous amount of fiber or wire to be installed and the design of hub devices that could support more locations than the ones presently available in the marketplace support. Also, the present invention includes field locations that are directly connected to the central location or host computer through a fiber optic cable or equivalent.
To present simultaneous communications to the field devices a special module was developed, as part of the present invention. The module provides for a type of drop and insert technique with two fiber receiving devices and two fiber transmitting devices on each module. Data sent from the central location or from another device in the chain is received by one or the other fiber optic receivers. Regardless of where the transmission originated, the central location or another device, it is sent to the regeneration circuit where it is passed on to other devices or back to the central location server.
When the data is received by the intended module, it is recognized and acted upon by the microprocessor. If the message demands a response, the data is assembled, the data switch is placed in the proper state, then the data is transmitted back to the central location server. This technique allows transmitted data to be passed on to other devices on a bit by bit basis. It also restores the optical power levels between devices.
One of the preferred embodiments for the present invention contemplates controlling 2,600 intersections or vehicular traffic control devices, and up to 8,192 other devices, such as, video cameras for surveillance and detection, variable message signs, highway ramp of control devices, information kiosks, etc., over more than a 750 square mile area. Control and video signals are transmitted, the latter requiring a broader bandwidth than the former. Also, future signal standards promise to be even more complex, increasing the desirability of the present invention.
One of the closest references correspond to U.S. Pat. No. 4,063,220 issued to Xerox Corp. However, it differs from the present invention in that a polling server with non-deterministic networked clients is not disclosed. The ability to simultaneously broadcast and the inherent propagation delays of the wide area network will make it impossible to use the method and apparatus disclosed by the patentee.
In fact, the patentee acknowledges the detrimental effect of using long cables for the communicating medium and thus documenting this limitation for large areas.
Other patents describing the closest subject matter provide for a number of more or less complicated features that fail to solve the problem in an efficient and economical way. None of these patents suggest the novel features of the present invention.
It is one of the main objects of the present invention to provide a network data communication system for communicating over a relatively large physical area permitting the connected stations to broadcast when requested by a polling communication server, thus reducing the probabilities of collisions and related delays.
It is another object of this invention to provide a high bandwidth network that permits the connected stations to transmit only when accessed by the server.
It is still another object of this invention to provide a network data communication system that is non-deterministic and its operation independent of network propagation delays.
It is still another object of this invention to eliminate the need for expensive single mode fiber optic modems that are presently used for data transmission in traffic control and other supervisory control and data acquisition (SCADA) systems.
It is yet another object to provide a system that utilizes commonly available components, such as Ethernet chip sets, to reduce the construction cost.
Another object of this invention is to port this technology onto government and privately owned copper cable plants to provide high speed communications at the cost of conventional low and medium speed modems.
With the above and other related objects in view, the invention consists in the details of construction and combination of parts as will be more fully understood from the following description, when read in conjunction with the accompanying drawings in which:
FIG. 1 represents a block diagram of a typical Ethernet LAN (local area network) installation.
FIG. 2 represents a block diagram of the main hardware used in one of the preferred embodiments for the present invention.
FIG. 3 shows a block diagram of the transceiver module incorporated on the vehicular traffic control stations.